The tall, columnar shape of the saguaro cactus is familiar to any desert traveler. But very rarely (about only one plant in 10,000), a saguaro will grow with a fan-shaped crest of uncontrolled growth at the tip. These oddities of the plant kingdom are known as cristate (or crested) saguaros. The deformity also occurs in other species of cactus, such as the organ pipe and barrel cactus. Science has not yet fully explained why this mutation occurs; perhaps it is caused by frost damage, although many other explanations have been offered.
If you ever happen upon one of these, remember that you are seeing something rare and special.